1. Field
The present disclosure relates to the field of photonics. In particular, it relates to broadband linearization of photonic modulation using transversal equalization.
2. Description of Related Art
In both terrestrial and satellite systems, there are substantial interests in using photonic links for signal distribution in antenna systems. For example, wavelength division multiplexed (WDM) technologies and analog photonic links for antenna remoting are currently used in a variety of space-based and terrestrial platforms. In all these applications, the link's spur free dynamic range (SFDR)—a parameter directly related to the linearity of photonic modulation at the transmitter end—is an important figure of merit for meeting the antenna system's performance goals. Additionally, photonic links are also used in so called fiber-radio networks, which have a significant commercial potential for distributing microwave signals between the base station of a wireless network and the remote sites of transmit/receive antennas.
The SFDR of analog photonic links can be enhanced by linearization approaches such as 1) negative feedback, 2) pre-distortion, or 3) feedforward error correction. The suppression of harmonic distortion in electronic amplifiers via negative feedback is well described in seminal papers/texts that discuss the design of feedback circuits.
Negative feedback for electronic amplifiers is discussed, for example, in “The Art of Electronics” by P. Horowitz and W. Hill, Cambridge University Press, 1989. Predistortion is discussed, for example, in R. Sadhwani, J. Bassak, B. Jalali, “Adaptive Electronic Linearization of Fiber Optic Links,” Paper ThG7, Optical Fiber Conference (OFC) 2003, Atlanta, Ga., Mar. 23-28, 2003.
For the broadband linearization of optical links, a feedforward scheme has the advantage that it does not incur design constraints for the time-delay generated in the feedback path, as in a negative feedback approach. The feedforward approach can also operate over a wider bandwidth than pre-distortion schemes because it does not rely on the use of nonlinear circuit elements to compensate—over a broadband—nonlinearities present in the modulation transfer curve.
The feed-forward approach was originally developed for linearizing electronic amplifiers, especially high power amplifiers and traveling wave tubes. Its application to photonic modulation was first reported in R. M. de Ridder, K. Korotky, “Feedforward compensation of integrated optic modular distortions,” Paper WH5, Optical Fiber Conference, 1990 and later in M. Nazarathy, J. Berger, A. Ley, I. Levi and Y. Kaggan, “Progress in Externally Modulated AM CATV Transmission Systems,” J. Lightwave Technol., vol. 11, no. 1, pp. 82-105, 1993. In particular, the Nazarathy paper mentioned the importance of maintaining a flat frequency response for all components (photonic and electronic) used in the feedforward path.
However, feedforward schemes for wideband distortion suppression are usually hampered by the existence of RF-ripples (for both amplitude and phase) in the feed-path to the linearizer. These non-idealities in the feedforward path can reduce significantly the null-depth of distortion cancellation, because they affect the designed balance in amplitude and phase between the signal-arm (where the signal travels) and the error-arm (where the distortion correction signal is generated). The Nazarathy reference mentions the importance of maintaining a flat frequency response for all components (photonic and electronic) used in the feedforward path, but offers no solution for broadening the effective bandwidth of the linearizer.